In evolutionary terms, homosexuality presents something of a paradox. According to Darwin, any trait that makes an animal less likely to reproduce will die out in a few generations, yet the percentage of people born gay or lesbian remains more or less constant.
Now researchers at the University of Portsmouth believe they may have found the evolutionary reason for homosexual behaviour: it helps us bond with people of the same sex.
A study of predominantly heterosexual men and women found that people with higher levels of the hormone progesterone are more likely to have homoerotic thoughts.
Because progesterone, which is produced by both men and women, is associated with affiliation, the researchers concluded that homosexual thoughts can go hand in hand with the need to forge same-sex alliances, which can be traced back to the teamwork of the earliest hunter-gatherers.
Dr Diana Fleischman, the report’s author, said: “In the paper we talk about why homosexuality persists, and we do explain why.
“From an evolutionary perspective we tend to think of sexual behaviour as a means to an end for reproduction.
“However, because sexual behaviour is intimate and pleasurable, it is also used in many species, including non-human primates, to help form and maintain social bonds. We can all see this in romantic couples who bond by engaging in sexual behaviour even when reproduction is not possible.
“Having some degree of attraction to the opposite sex is a type of adaptive behaviour, and in any adaptive behaviour you will see extremes of the spectrum, hence some people will only be attracted to members of the same sex.
“But the research suggests that having exclusively heterosexual thoughts is a disadvantage – it’s better to be a little bit attracted to the opposite sex.”
Dr Fleischman said her research did not point to any correlation between environmental levels of progesterone and sexual orientation. She said that while a synthetic form of progesterone is used in the contraceptive pill, which then enters drinking water supplies, “we didn’t find any difference between women on the pill and women not on the pill”.
She added that while environmental levels of progesterone have increased, there is no evidence that the percentage of people who are gay or lesbian has gone up.
Gerard Conway, Professor of Reproductive Endocrinology at University College, London, agreed, saying: “The idea of hormones from the contraceptive pill permeating drinking water is an urban myth. The amounts are so tiny you can’t even measure it. Progesterone is just not on the radar as an environmental toxin.”
Dr Fleischman’s study, published in the journal Archives of Sexual Behaviour, asked participants to answer questions such as: “The idea of kissing a person of the same sex is sexually arousing to me,” and: “If someone of the same sex made a pass at me I would be disgusted.”
By comparing their answers with the progesterone levels in their saliva, the researchers were able to establish an apparent link between progesterone and homosexual thoughts.
Progesterone is produced mainly in the ovaries in women and in the adrenal glands in men. It is one of the main hormones responsible for caring or friendly behaviour and levels rise when people have close and friendly interactions.
It does not, however, increase sexual desire; sex offenders are sometimes treated with progesterone to quell their sexual urges.
Dr Fleischman said that studies of other animals in the great ape family also pointed to homosexual behaviour being used to maintain and forge new friendships.
She said: “The ability to engage sexually with those of the same sex or the opposite sex is common. In humans, much, if not most of same-sex sexual behaviour occurs in those who don’t identify as homosexual.”
Prof Conway, however, questioned whether the research had proved anything.
He said: “It is a plausible theory that there is a societal benefit from homosexual behaviour, but the link to progesterone is probably spurious. It’s a long way from proving cause and effect.”